Current strict low emission vehicle regulations in each country request pretty high injection accuracy in each fuel injection. Specifically, recent diesel engines are requested to perform pilot-injections or multi-injections in accordance with the strict low emission vehicle regulations, so that it is required to increase an injection accuracy of each fuel injection. However, manufacturing tolerances and/or secular changes occurring in the injector may change injection amount and/or injection timing. Thus, it is requested to develop an injector maintaining high injection accuracy over a long period of usage.
In the following is described an example in which the manufacturing tolerances and/or secular changes spoil a fuel injection accuracy of the injector.
FIG. 5 schematically depicts a structure of a conventional injector 3 (refer to U.S. Pat. No. 6,698,666-B and its counterpart JP2003-97378-A, for example). The injector 3 has a fuel inflow passage 31, a fuel discharge passage 32, a control chamber 33, a control valve 34, a command piston 35, a needle 36, a housing 38 and a nozzle chamber 44. The housing 38 supports the command piston 35 and the needle 36 to allow a reciprocating motion therein. The housing 38 and the command piston 35 enclose the control chamber 35 therebetween to define an outline thereof. High-pressure fuel is introduced through the fuel inflow passage 31 into the control chamber 33. The high-pressure fuel accumulated in the control chamber 33 is discharged through the fuel discharge passage 32. The fuel discharge passage 32 is blocked and opened by the control valve 34, which is actuated by an electric valve such as an electromagnetic valve. The nozzle chamber 44 is disposed around the needle 36, and a high-pressure fuel is supplied thereinto to push the needle 36 in a valve-opening direction.
As shown in FIG. 2A, when the injector 3 opens, the electromagnetic valve is turned on to draw up the control valve 34 to open the fuel discharge passage 32. Then, a piston control pressure Pcc, which is a pressure exerted by the high-pressure fuel in the control chamber 33 on the command piston 35 in an axial direction of the injector 3, decreases from a common rail pressure Pc to a valve-opening pressure Popn; thereby a conically-shaped needle head 36a lifts off the needle seat 45, which is formed in the housing, to start injecting the high-pressure fuel through the injection holes 46. It takes a time (hereinafter referred to as an injection start delay) Tds from turning on the electromagnetic valve to the fuel injection start by a decrease of the piston control pressure Pcc below the valve-opening pressure Popn.
That is, the command piston 35 receives the piston control pressure Pcc in a valve-closing direction (downward in FIG. 1). The needle 36 receives a counter-pressure Pc in a valve-opening direction (upward in FIG. 1). The counter-pressure Pc is approximately equal to the common rail pressure Pc. Thus, in order to start fuel injection by the injector 3, a pressure difference (Pc−Pcc) must be over a valve-opening pressure difference dP0. Thus, in order to start fuel injection by the injector 3, it is necessary to decrease the piston control pressure Pcc below the valve-opening pressure Popn so that the pressure difference dP0 (Pc−Pcc) becomes over the valve-opening pressure difference dP0.
In simple explanation to disregard a valve return force exerted by a valve return spring on the command piston 35 in the valve-closing direction, the piston control pressure Pcc exerts a valve-closing force on the command piston 35 as much as a product (Pcc×Scc) of the piston control pressure Pcc and a pressure-receiving area Scc on an upstream end face of the command piston 35. The counter-pressure Pcc exerts a valve-opening force on the command piston 35 as much as a product (Pc×Snc) of the counter-pressure Pc and a pressure-receiving area Snc on a downstream end face of the command piston 35. Thus, if manufacturing tolerances and/or secular changes occur in a diameter Dns of a needle seat portion 47, the pressure-receiving area Scc changes, thereby the above-described valve-opening force also changes. Specifically, the valve-opening pressure Popn decreases to Popn′ as shown in FIG. 2A. Accordingly, in order to start fuel injection by the injector 3, it is necessary to adjust the piston control pressure Pcc.
A change of the valve-opening pressure from Popn to Popn′ further changes the injection start delay from Tds to Tds′. That is, if the diameter Dns of the needle seat portion 47 includes a relatively large tolerance or error, the injection start delay changes from Tds to Tds′, so that a target injection amount Q0 and a target injection timing T0, which are calculated in accordance with a current driving condition, include errors to spoil a high accuracy in fuel injection deviated from ideal values thereof.
When the injector 3 is closed to stop fuel injection, as shown in FIGS. 6A and 6B, the needle head 36a is apart from the needle seat 45, so that the valve-closing timing is not deviated by a change of the diameter Dns of the needle seat portion 47. That is, the valve-closing timing is not affected by the manufacturing tolerances and/or secular changes occurring, which may occur in the diameter Dns of a needle seat portion 47.